[Cryptography] The ultimate random source

[Peter Gutmann]

Christian Huitema <huitema at huitema.net> writes:

>If you rely on a capped camera to generate white noise, you may be out for a
>surprise with at least some cameras. There is a lot of filtering and
>processing that happens on board the camera itself, e.g. conversion from
>Bayer pattern to YUV or RGB, firmware that enhances the image, compression to
>JPEG before transmission on the USB bus, cropping and resizing on demand, etc.
>I would not be surprised if some cameras, when capped, just transmit a black
>image.

I would certainly hope they transmit as close to a black image as possible,
since they're designed to do that.  For example when shooting dark scenes some
(still-image) cameras will take a second image with the shutter closed and use
that to get an idea of any noise present, then use that information to cancel
noise in the primary image.  This will remove a lot of the hoped-for noise. In
addition since a lot of what you're hoping for is thermal noise, taking the
images infrequently (so the sensor is relatively cool, therefore with low
noise), or with the camera in a cold room, is going to reduce noise
significantly.  This is why devices like telescopes and medical imaging
devices use externally-cooled CCD sensors, I've seen serious (or perhaps
slightly crazy) people pull apart thousand-dollar DSLRs to fit TEC (Peltier)
elements to them.

I've already said this before, but I'll say it here again: The most you can
say about sensor noise (optical, audio) is that any value you get for your
particular setup applies only for that specific combination of hardware,
software, and environmental conditions.  These devices are specifically
designed to get rid of as much noise as is physically possible, so can't be
relied on, in isolation, as noise sources.

Peter.